Conventional two cycle, spark-ignited engines have a reputation for erratic operation at light loads. If the engine is highly throttled the residual fraction becomes very large (70 to 90 percent of the trapped charge), excessively diluting the fresh charge and resulting in misfires or incomplete combustion. If the engine is not throttled the fuel-air ratio is outside the bounds of ignitability. Even adding direct fuel injection, in an attempt to achieve charge stratification, does not permit the two cycle engine to accommodate light loads as well as the four cycle engine.
Such conventional two cycle engines generally employ a scavenging system which includes piston-controlled intake and exhaust ports and air supplied by crankcase compression. When the piston moves upward on the cylinder compression stroke, it increases the volume of the crankcase, lowering the pressure, and causes atmospheric air to enter the crankcase through an intake system. The intake system includes a throttle for flow control and, downstream close to the crankcase, a set of reed check valves which automatically open inwardly to the crankcase in response to a depressed crankcase pressure allowing fresh charge to enter. When the piston moves downward on the cylinder expansion stroke, the volume in the crankcase is reduced, the reed valves automatically close preventing gas escape from the crankcase, and the pressure is increased until the intake port is uncovered by the piston. Then fresh charge from the crankcase enters the combustion chamber forcing the products of combustion out of the chamber through the exhaust port.
At maximum load, corresponding to wide-open throttle, the pressure drop across the reeds reduces the mass of fresh charged delivered to the engine. Furthermore, if crankcase pressure becomes equal to atmospheric pressure during upward piston motion prior to intake port closure, the fraction of reverse flow through the transfer ports will be decreased if the resistance to flow offered by the reed valves can be decreased.
Part-load operation is achieved by closing the throttle until the fresh-charge mass flow rate matches that required by the engine load demands. This depresses the crankcase pressure during the engine compression stroke and produces pumping losses. The processes within the crankcase are illustrated in FIG. 1 on a pressure-volume diagram where the net pumping work is represented by the shaded area.